In general, a single phase unidirectional induction motor frequently used for a compressor is provided with a stator, a rotor, and a running capacitor.
Referring to FIG. 1 illustrating a quarter of a motor stator section, the stator has slots 100 at fixed intervals. The stator has two windings each turned around the slots a few tens of times. The running capacitor is connected to one of the two windings in series. The rotor is rotated under the influence of a magnetic field formed by the stator.
A system and operation of the unidirectional motor will be explained, with reference to FIG. 2.
Referring to FIG. 2, the unidirectional motor is provided with two windings a–a′, and b–b′ and a running capacitor.
One end ‘a’ of the a–a′ winding and one end ‘c’ of the running capacitor are connected to a power source. The other end ‘c′’ of the running capacitor is connected to one end ‘b’ of the b–b′ winding. The other end ‘a′’ of the a–a′ winding, and the other end ‘b′’ of the b–b′ winding, are grounded.
Currents are provided to the two windings, with directions of the currents fixed beforehand, such that the rotor is rotated in a required direction according to Flemming's left-hand rule. In this instance, the current to the winding b–b′ having the running capacitor connected in series has a phase 90° earlier than the winding a–a′ having no running capacitor connected thereto. Then, a magnetic field formed by the current to the winding b–b′ having the running capacitor connected in series produces a starting torque, and a magnetic field formed by the current to the winding a–a′ having no running capacitor connected thereto sustains the rotation of the initially started rotor. Eventually, the rotor of the unidirectional motor is rotated in a required one direction under the influence of the magnetic fields formed by the currents to the two windings.
The running capacitor, a kind of starting means for starting the rotor, can be replaced with a switch, or a combination of the running capacitor and the switch, though a principle for starting the rotor is a little different from the running capacitor.
In the meantime, a winding ratio (‘a number of wires passed through a section of the slot having no running capacitor connected thereto in series’/‘a number of wires passed through a section of the slot having the running capacitor connected thereto in series’), and a sectional area ratio (‘a sectional area of a winding having no running capacitor connected thereto in series’/‘a sectional area of a winding having the running capacitor connected thereto in series’) influence the starting torque of the motor and a motor efficiency (‘work done by the motor’/‘power supplied to the motor’). The winding ratio and the sectional area ratio of the unidirectional motor are designed so that the unidirectional rotation of the motor is appropriate, in other words, the starting torque is adequate for the unidirectional rotation, and an efficiency of the motor is maximized. In general, the winding ratio of the unidirectional motor is set to be within a range of 1.1–1.6, and the sectional area ratio is set to be within a range of 1.44–2.56.
Thus, because the related art unidirectional motor requires a separate mechanical device, such as a clutch, for reverse direction rotation, the related art motor has a high production cost, a greater volume as much, and noise from the mechanical device.